1. Technical Field
The present invention generally relates to pipette tips that displace relatively small volumes of liquid sample during pipetting such that the tips may be used in single channel or multichannel applications, and more specifically to such pipette tips that have relatively large volume capacities.
2. Discussion of the Prior Art
Pipette tips have been used to manipulate liquid samples for decades. Generally, the particular design and dimensions of a pipette tip depend upon the intended use of the pipette tip. For example, a pipette tip fabricated for pipetting small volumes of liquids may be designed to have minimal interior dead air space while a pipette tip fabricated for use with filters may have a large interior dead air space. In certain circumstances, it is desirable to provide a pipette tip that may be used with conventional microtiter tubes, standard sized test tubes and microtiter plates. However, pipetting operations conducted with microtiter tubes and test tubes are normally carried out using single channel pippetors while pipetting activities conducted with microtiter plates are typically performed using multichannel pipettors. Thus, a pipette tip that is designed for use with microtiter tubes, test tubes and microtiter plates should be compatible with both single channel pipettors or multichannel pipettors.
Pipette tips designed for pipetting large volumes of liquid (i.e., volumes greater than about 625 .mu.L) are commercially available in a variety of designs. FIG. 1 is representative of one such pipette tip 10 that is designed for use in single channel or multichannel applications with commercial pipettors, such the as Impact.RTM., Impact2.RTM., Micronic.RTM. and/or Electrapette.RTM. pipettors (each a registered trademark of Matrix Technologies Corporation, located in Lowell, Mass.).
Prior art tip 10 is a unitary molded piece having a head 12, a midsection 14 and a stem 16. Tip 10 further includes a passageway 18 which extends longitudinally from opening 20, passing entirely through tip 10 and terminating at orifice 22. Tip 10 is designed such that the volume of opening 18 is about 1250 .mu.L. Therefore, the maximum outer diameter and length of midsection 14 are about 0.24 inches and about 1.2 inches, respectively, and the maximum outer diameter and length of stem 16 are about 0.24 inches and about 0.75 inches, respectively. In addition, the maximum inner diameter of orifice 22 is about 0.05 inches.
While tip 10 may operate adequately with microtiter plates, FIGS. 2 and 3 show tip 10 partially disposed within a conventional microtiter tube 24 and a standard sized test tube 23, respectively. Microtiter tube 24 has a length of about 1.7 inches and an inner diameter of about 0.27 inches, and test tube 23 has a length of about 3.9 inches and an inner diameter of about 0.5 inches. Tip 10 may be capable of accessing the bottom of microtiter tube 24, but the comparatively large combined volume of midsection 14 and stem 16 results in a substantial amount of liquid sample being displaced as the sample is pipetted from microtiter tube 24. This excessive sample displacement during use with microtiter tubes is disadvantageous because it results in sample loss and possible cross-contamination between samples. Furthermore, although tip 10 may not displace an excessive amount of sample volume from test tube 23, tip 10 is unable to access the bottom of tube 23. Hence, tip 10 cannot be used effectively with certain conventional microtiter tubes and standard sized test tubes.
While holding the volume capacity of tip 10 constant at about 1250 microliters, the sample displacement caused by tip 10 can be reduced by decreasing the combined volume of midsection 14 and stem 16 while keeping the volume of opening 18 constant. This may be achieved by increasing the length of stem 16, which would also allow tip 10 to access the bottom of test tube 23. However, if stem 16 is too long, it may not remain straight, compromising the accuracy of tip 10. Moreover, in certain cases, it may be advantageous for a technician using tip 10 to have control of droplets formed at orifice 22, such as when "touching off," a procedure in which the technician removes the droplets from orifice 22. However, increasing the length of stem 16 can reduce the control a technician may have over these droplets. Also, if stem 16 is too long, an operator may generally lose control of tip 10, causing the operator to use two hands to steady tip 10. Therefore, the length of stem 16 cannot be increased to an arbitrarily high value. Furthermore, tip 10 should be capable of housing a filter to reduce aerosol contamination and/or to prevent the liquid sample from entering the pipettor. Such a filter is typically contained within midsection 14, so the volume of midsection 14 cannot be reduced below the volume of the filter. In addition, to improve the accuracy of tip 10 by minimizing droplet formation at orifice 22, the maximum inner diameter of orifice 22 should be minimized. Thus, a careful balance of a variety of the dimensions of a pipette tip must be reached to provide a pipette tip with a volume of about 1250 .mu.L that is compatible with microtiter tubes, test tubes and microtiter plates.
Presently, it remains a challenge in the art to provide such a pipette tip. Furthermore, due to the delicate balance of parameters involved as described above, when preparing this pipette tip, one skilled in the art would not simply look to the many different available pipette tip designs that are known and select particular dimensions from these pipette tips in an ad hoc fashion.